feat: add functionality to manually split documents at specific pages via the UI and API

paperless-backend/src/database/entities/inbox-document.entity.ts

@@ -59,6 +59,16 @@ export class InboxDocument {
   })
   Rotations!: Record<string, number>;
 
+  @Column({
+    type: 'json',
+    nullable: true,
+    transformer: {
+      to: (v: number[] | null | undefined) => (v && v.length ? v : null),
+      from: (v: number[] | null) => v ?? [],
+    },
+  })
+  ManualSplitPages!: number[];
+
   @CreateDateColumn()
   CreatedAt!: Date;